Surface Plasmon Resonance (SPR) relates to optical excitation of a surface plasmon wave along an interface between a conductive film and an adjacent dielectric. At resonance, energy from an incident optical signal is coupled into the surface plasmon wave, resulting in a decrease in the intensity of the optical signal that is reflected from the conductive film at the optical wavelength at which the resonance occurs. A phase transition in the reflected optical signal also occurs at the resonant optical wavelength. The phase of the reflected optical signal at the resonant optical wavelength is sensitive to changes in the refractive index of the dielectric that is adjacent to the conductive film, due to an evanescent tail associated with the surface plasmon wave that extends into the dielectric. This phase sensitivity to refractive index enables the dielectric to be used as a sensing medium. For example, where the dielectric includes a biochemical sample, changes in the refractive index can be used to indicate biochemical conditions or processes in the biochemical sample.
Due to the sensitivity of phase transitions in the reflected optical signal to changes in refractive index of the dielectric or sensing medium, phase measurements based on SPR have been exploited as an analytic tool for biochemical sensing. For example, Immunosensor based on optical heterodyne phase detection, Sensors and Actuators B, vol. 76, Xinglong Yu, et al., (2003), p. 199–202, reports interferometry, optical heterodyning and other techniques to detect phase changes corresponding to smaller than micro-unit changes in refractive index that are linked to association/dissociation of antibodies and a protein in a biochemical sample.
There is a continuing need for phase detection schemes to further increase the sensitivity with which changes in refractive indices can be detected. In addition, there is a need for phase detection schemes based on SPR that are scalable for compatibility with analytical systems that include arrays of samples for biochemical sensing.